The following are circuit fragments. They are only building blocks, not complete effects. Also note that I am really a software guy, so these fragments may not work under all conditions. Use at your own risk.
This is a very clean, very low power unity gain input buffer. I measured the current draw at 46uA. The input impedance, as shown, was measured at 414k @ 100Hz. I have tested replacing the two 1M resistors with 2.2M for higher input impedance and it works.
The 1k and .001uF resistor and cap at the input are optional. They help to reduce noise.
This is a very clean, low gain, fragment that I have used mainly as an input buffer. It has about 6dB of gain and requires very little power.
It was designed specifically for the 2N5458 but you can try other j-fets. It should keep the 2n5458 well within its design limits and work out of the box with most 2N5458s. It was designed as an input stage, so to keep it clean you must keep the input voltage with 1V or so.
This is a lower gain, higher input impedance version of Gus Smalley's NPN Booster. It uses "input bootstrapping" to increase the gain. I measured 2M at min gain and 1.3M at max gain. The gain control affects the impedance. If you replace the 4k7 and 5k pot with a 10k pot, impedance at max gain drops to 124k.
The original tends to roll off the low frequencies. This version tends to work better for bass where the extreme gain is not needed.
The 100k output resistor can be replaced with a 100k pot for a volume control.
This is the first preamp stage for a Bassman amp. Input impedance is approximately the gate resistor value. Very clean depending on choice of Q1.
I also show the bright input channel from the original schematic for comparison. The other 1/2 of the 12AY7 handled the normal channel which was a mirror image except for the .001uF cap. For those not used to tube circuits, the number after a cap value is the voltage. The original schematic comes from Ampwares "The Fender Amp Field Guide" a great site with a lot of Fender amp history, schematics, and more!
Q1: I used a 2N5458. I also tried the J201 which seemed darker and started to distort at surprisingly low gains. The 2N5458 remains clean. The 2N5458 gave about 6x gain, the J201 10x.
Bias Trimmer: The bias trimmer needs to be at least 5k for a 2N5458 and at least 15k for a J201.
820Ω: The 820Ω resistor has the Bassman mojo, but I found 1k actually works better.
250uF: Works just as well with a 10uF. Leave out completely for less gain. Interestingly, values > 10uF seemed to lower the gain until it can back at about 200uF.
Gain mod: The 820Ω resistor
can be replaced with a 1k pot as shown. This will allow the gain
to be adjusted from about 2x to 6x with an 2N5458.
Bright: The bright switch and 100pF cap are optional. They
mimic plugging into the bright input.
Contact Sean MacLennan
Last modified: Thu Oct 19 21:07:29 EDT 2006